This invention relates to improvements in the construction of electric heaters of the type including a heating plate formed by a steel plate coated with a dielectric insulator, preferably vitrified enamel, such plate having on one of the faces thereof a printed circuit, thus forming an electrical resistance which, when connected to a feed network, emits a focused heat.
Heating plates of this type which presently exist in the market consist of a mere flat plate, generally rectangular in shape, the heating of which, when in operation, gives rise to certain disadvantages, the most serious being uncontrolled deformation of the plate due to expansion and contraction thereof produced when the plate is successively heated and cooled.
The deformation of the plate itself produces a series of disturbing noises which are objectionable.
In order to avoid both the uncontrolled deformation of the plate and the emission of noises during operation of the heater, the radiant plates are firmly fixed to a frame which conforms in shape to the radiant plates, but which, nevertheless, does not prevent the appearance of such harmful and annoying phenomena.
The uncontrolled deformation of the plate subjects the porcelain-like dielectric coating to bending stresses, produced by the twisting of the plate, which are not withstood by such coating. Thus, crevices or cracks, commonly known as "hair lines," are produced. Under such circumstances the printed circuit becomes connected to the frame and the heating plate then becomes useless.
Logically, the twisting or uncontrolled deformation of the plate is a direct function of the temperatures produced by the printed circuit when heated. Therefore, it can be affirmed that the admissible power limit for a flat plate has a value of approximately 0.4 watts/cm.sup.2, which provides an average surface temperature in the range of about 170.degree. C.